Field of the Invention
The present invention relates to a reinforced composite membrane for fuel cells and a membrane-electrode assembly for fuel cells comprising the same which exhibit improved proton (hydrogen ion) conductivity owing to increase in impregnation uniformity and impregnation rate of an ionic conductor.
Description of the Related Art
A fuel cell is a battery which directly converts chemical energy generated by oxidization of fuels into electrical energy and attracts much attention as a next-generation energy source due to high energy efficiency and eco-friendliness associated with less contaminant discharge. Such a fuel cell generally has a structure in which an electrolyte membrane is disposed between an anode and a cathode.
Representative examples of fuel cells for vehicles include proton exchange fuel cells which employ protons (hydrogen gas) as a fuel. The electrolyte membrane used for the proton exchange fuel cell should basically have superior proton conductivity because it serves as a passage enabling hydrogen ions (protons) generated in an anode to be transported to a cathode. In addition, the electrolyte membrane should satisfy requirements such as superior capability to separate hydrogen gas supplied to the anode and oxygen supplied to the cathode, and excellent mechanical strength, mechanical strength, shape stability, chemical resistance and the like, and less resistance loss at high current density. In particular, fuel cells for vehicles should have superior heat resistance so as to prevent the electrolyte membrane from being broken, when used at a high temperature for a long period of time.
As a commonly used electrolyte membrane for fuel cells, there is a perfluorosulfonic acid resin which is a fluorine-based resin (Nafion® (hereinafter, referred to as a “Nafion resin”). However, the Nafion resin has problems of low mechanical strength, generation of pin holes when used for a long time and thus decreased energy conversion efficiency. In an attempt to reinforce mechanical strength, a Nafion resin with an increased membrane thickness has been used. In this case, problems such as increased resistance loss and decreased economic efficiency caused by use of expensive materials occur.
A reinforced composite membrane was suggested in order to solve these problems and remedy drawbacks of conventional electrolyte membrane. The reinforced composite membrane includes an ionic conductor as an electrolyte substance and a porous support to remedy drawbacks of single electrolyte membranes, such as dimensional stability, durability and mechanical strength. A representative product including reinforced composite membrane is PRIMER® (produced by GORE-TEX Inc.). The reinforced composite membrane improves mechanical strength by combining a porous polytetrafluoroethylene resin with a fluorine-based ionic conductor. However, use of the fluorine-based ionic conductor and the fluorine-based porous support disadvantageously entails low competitiveness in need of price reduction in order to commercialize fuel cells.
Accordingly, low-cost hydrocarbon-based ionic conductors as substitutes for high-cost fluorine-based ionic conductors were developed, but the hydrocarbon-based ionic conductors are disadvantageously unsuitable for application to fluorine-based porous supports.
Accordingly, hydrocarbon-based porous supports suitable for hydrocarbon-based ionic conductors and more specifically, porous supports which may be used as composites with fluorine-based ionic conductors, as well as the hydrocarbon-based ionic conductors, are required. Furthermore, a reinforced composite membrane is used for fuel cells as a membrane-electrode assembly in which electrodes are bonded to both surfaces of the reinforced composite membrane having superior dimensional stability. Accordingly, a reinforced composite membrane having excellent dimensional stability is needed in order to prevent deterioration in performance and durability of fuel cells caused by contact defects on the interface between the electrodes and the reinforced composite membrane.